1. Field of the Invention
The present invention relates to the transmission of digital data over a low frequency A.C. supply line, for example, the 50-Hz or 60-Hz mains voltage.
The present invention more specifically applies to the transmission of digital data between a transmitter placed on an A.C. supply line dedicated to a load and this load. This line can be, for example, the supply line between a switch and a lamp (for example, a ceiling lamp), between a switch and a controlled tap, between a control station and one or several electric devices that it controls by powering them and, more generally, between any device of remote control and a load to which the control device is connected by an A.C. low frequency supply line.
2. Discussion of the Related Art
FIGS. 1 and 2 illustrate a technique conventionally used when a digital information is desired to be transmitted over a low frequency A.C. supply line. This technique consists of using the so-called carrier current principles, that is, using the A.C. supply voltage as a transmission support for a low voltage signal superposed to the A.C. voltage.
Implementing such a solution requires (FIG. 1) a system 1 of modulation (Tx) adapted to modulating A.C. supply voltage Vac (for example, the mains voltage) to provide an A.C. voltage Vm (FIG. 2) over the supply line (phase 2, neutral 3) of a load (Q). On the side of load 4, a demodulation system 5 adapted to extracting the modulating signal from the carrier current has to be available.
The transmission of digital information on a carrier current has to deal with significant constraints which result in the use of particularly complex transmission/reception systems to succeed in properly exploiting the digital data.
First, the system depends on the impedance of the supply main. Due to the low impedance of electric lines, signals transmitted in modulation have to exhibit the lowest possible energy level and are thus difficult to recover on the receive side.
Further, the recovery of the data signals requires a selective filtering to separate the data from the low frequency A.C. voltage and from its disturbances.
In some applications, the constraints linked with carrier current data transmission cause the return to a still prior solution which consists of using an additional link dedicated to the data transmission. Such an additional link has the obvious disadvantage of multiplying the number of live wires between the transmitter and the receiver, and of being impossible to use in an existing installation where a single twin-wire A.C. supply line connects the transmitter to the load.
To avoid such an additional link, European Patent Application 0,038,877 provides for isolating the supply line from the load so that the conductors of the line can be used to transmit a signal other than the supply signal. A drawback of this approach is that the power supply is interrupted during the whole duration of the information transmission. Additionally, it is necessary not only to isolate the power supply line but also to connect to this line, during the transmission periods, an emitter generating the information signal and its carrier.
Another approach consists in interrupting the load supply line in according with the binary coding of an information to be transmitted, each period of the A.C. power supply containing at most one bit. For example, the International Patent Application 93/18568 provides for interrupting the line during the half of a positive halfwave to transmit a xe2x80x9c1xe2x80x9d, a xe2x80x9c0xe2x80x9d being represented by a non modified period of the A.C. supply. The transmission rate corresponds to the frequency of the A.C. supply. A similar solution is disclosed in the United Kingdom Patent Application 2,298,553 that provides for interrupting the supply of a lamp at each positive halfwave (at most once per halfwave) of the A.C. supply voltage. A drawback of these solutions, in addition to a very low rate, is that they impair the supply of the load which no longer receives a supply during the periods during which it needs energy.
The present invention aims at providing a novel solution to transmit digital data over an A.C. supply line.
The present invention especially aims at providing a transmission of digital data which can be performed by means of transmitters and receivers of particularly simple structure while maintaining the load supply.
The present invention also aims at overcoming the disadvantages of conventional methods of transmission by carrier current modulation.
The present invention also aims at providing a solution in which the transmission rate is not linked with the load supply frequency, and that permits higher rates than the rates of the know solutions that interrupt the lines in accordance with the data flow to be transmitted.
A feature of the present invention is to dissociate in time, in the A.C. signal, periods when the A.C. voltage has to supply the load and periods where this A.C. supply is not exploited by the load and can thus be used to carry digital data.
Thus, conversely to conventional methods, which consist of superposing the power signal and the data signal, the present invention provides to have the load supply periods and the data transmission periods follow one another in time.
Another feature of the present invention is to use the A.C. supply voltage as an envelope signal for the digital data during periods where said data are present.
Thus, the present invention provides to periodically transfer the power required to supply the load and to use the periods which are not used by this load supply to transmit the digital data by switching the A.C. voltage at a frequency greater than the frequency of the A.C. voltage.
Generally, the present invention is based on an observation made by the inventors according to which, when data are desired to be transmitted to a load by the carrier current technique, this load very often has a capacitive input impedance or, more generally, does not permanently take power from the mains. In particular, when digital information are desired to be transmitted, the load is most of the time supplied from a substantially D.C. voltage, generally originating from a rectification, even though this D.C. voltage then is turned back into a discontinuous voltage (for example, by a switched-mode converter). As a result, the load is often associated with a storage means of power that it only cyclically takes from the mains.
More specifically, the present invention provides a method of data transmission over an A.C. power supply line of a load to be cyclically powered, including organizing a switching of the A.C. supply voltage according to a coding of the data to be transmitted and outside cyclic load supply periods, using the A.C. supply voltage as an envelop of pulses having a higher frequency.
According to an embodiment of the present invention, the frequency of the A.C. power supply is relatively low with respect to the relatively high switching frequency for the data transmission.
According to an embodiment of the present invention, the method includes organizing the switching of the A.C. supply voltage, every other halfwave.
According to an embodiment of the present invention, the method includes organizing the switching of the A.C. supply voltage at each halfwave, during the decreasing portion of the halfwave.
The present invention also provides a transmitter of digital data over an A.C. power supply line of a load to be cyclically powered, including a switching means adapted to interrupt the A.C. power supply line according to a coding of the digital data to be transmitted, using the A.C. supply voltage as an envelop of pulses having a higher frequency, the switching means being controlled outside the cyclic load supply periods.
According to an embodiment of the present invention, the transmitter includes a one-way conduction component in parallel with the switching means.
The present invention also provides a receiver of digital data transmitted over an A.C. power supply line of a load to be cyclically powered, the receiver including means for fetching the transmitted digital data from the load supply voltage and the data being formed by a switching, outside the cyclic load supply periods, of the A.C. supply voltage according to a predetermined coding.
According to an embodiment of the present invention, the receiver is associated with a capacitor adapted to providing an approximately D.C. voltage to a load supply circuit, and it includes a one-way conduction means adapted to isolating the capacitor from the means of extraction of the digital data.
The present invention further provides an A.C. load supply signal, including periods of load power supply and periods of transmission of digital data by switching of the signal at a frequency higher than the frequency of the A.C. signal.
According to an embodiment of the present invention, the frequency of the A.C. signal is low with respect to the switching frequency coding the digital data.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.